Shedding light on the mystery of the Ediacaran Fauna

kmd9
Monday 23 July 2018

Strange, extinct life always holds a certain fascination, of which the Dinosaurs are the best example. 500 million years before T-Rex however, a less fearsome but equally weird and fascinating group of life colonised the ancient sea floor. The “Ediacaran fauna” existed in a pocket of time 575 to 542Ma, following a series of ‘snowball earths’ and before the ‘explosion’ of predatory animals in the Cambrian. Evidence of these bizarre creatures, christened with names such as Pteridinium, Yorgia and Cyclomedusa, is preserved in rocks across the globe.

An artist’s impression of the Ediacaran sea floor

Frond like impressions, meandering traces and quilted blobs reaching up to 1m in diameter are typical Ediacaran fossils. What were they? Were they ancient algae, lichens, bacterial colonies or animals such as sponges, molluscs or an another group of life altogether? Did they absorb their nutrients through their skins or filter-feed like modern sponges? The identity and lifestyle of these strange creatures remains a mystery.

My project aims to shed some light on these questions. Most past arguments for the

Corumbella

identity of the Ediacaran fauna are based on what they looked like, analysing their morphology. My project aims to try a different tact, instead looking at what they were made of. Although most Ediacaran fossils are just impressions, a few appear to preserve traces of carbonaceous and other material from the creatures themselves.

Techniques such as laser ablation, Raman spectroscopy and X-ray diffraction may reveal if certain elements are concentrated in the fossil impression compared to the surrounding matrix. If for example high concentrations of Barium and Sulphur were found in the fossil traces, it could suggest similarity to xenophyophores, creatures which live on the deep sea bed with a strangeness to rival that of the Ediacaran fauna. Single cells are normally on the micrometer scale, yet these protists are orders of magnitude larger, reaching 20cm. They unusually contain high concentrations of the mineral barite (BaSO4), which has been hypothesised to allow such gigantism and keep them weighted to the sea floor.

Isotopic analyses could allow even more inferences. Carbon, essential for all life, exists in two stable isotopes 12C and 13C. Life uses the lighter isotope at a greater rate, thus biotic remains will have a different ratio of the two to the surrounding rock. The exact ratio is characteristic of different forms of life, distinguishing bacteria from animals for example. Sulphur and nitrogen isotopes may provide even more insights.

Putative Palaeopascichnus impressions

I have access to fossils of a few Ediacaran species, the most intriguing being a putative Palaeopaschinus fossil from the Wonoka formation in Australia. The fossil consists of pretty meandering patterns of small dark crescents on limestone . These appear to contain carbonaceous material (most Palaeopaschinids are found as solely impressions on sandstone, thus its identity is contraversial), thus should provide a good opportunity for isotopic tests.

I have just reached the end of the first week of my project. On the first day I was filled with a certain dread in knowing where to start and what to do, having never seen the specimens before and the array of possible tests being rather mysterious. After a discussion with my supervisor, I settled down to getting to know my samples ahead of the tests; taking photos, cleaning them up, sketching them,

Sketch of the fossil impressions

describing them and carrying a lot of rock up and down stairs. The Paleopaschinids were part of a 14 piece 3D rocky jigsaw which had to be solved, to give the location of the fossil bed in relation to the other beds (which were rather chaotic). It also allowed the discovery of more impressions. When likely candidates were washed with dilute acid, as if by magic more fossil traces emerged. Next week I have the promise of drills and rock saws to look forward to, and after that hopefully the geochemical tests will yield some interesting results.

Part of the Palaeopascichnus puzzle

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